1. Field of the Invention
This invention relates to the field of contact-free dimensional measurement in general. More particularly, the invention relates to a capacitive dimensional measurement chain designed to directly produce a measurement signal linearly representing a distance to be measured.
Within this sphere of the technique, there are principally two types of measurement devices in use at present:
impedance measuring bridges, and PA1 capacitive measurement chains.
Manual or automatic type impedance measuring bridges are awkward to use, have restricted resolution and can only be used in static operational conditions. Furthermore, they are costly and non-linear as regards dimensional measurement.
2. State of the Prior Art
A capacitive measurement chain such as those disclosed in French patent application No. 2,608,751 delivers a measurement signal representative of a capacitance. The measurement signal is inversely proportional to the distance to be measured and does not therefore represent this distance linearly. A digital or analog linearizing device is also included in the measurement chain so as to linearize the measurement signal. The linearizing device has the disadvantage of introducing additional errors, and consecutively reducing the efficiency of the dimensional measurement.
This disadvantage is avoided in the capacitance measurement chain described in U.S. Pat. No. 4,067,225. The chain comprises an oscillator providing an a.c. reference voltage and powering a standard capacitor. One output terminal of the oscillator is coupled through the standard capacitor to one input of a first amplifier and is coupled by an inner conductor of a coaxial cable to a central electrode of a capacitive sensor. Another output terminal of the oscillator is directly coupled to another input of the first amplifier and is coupled to a guard ring of the sensor through a shielding external conductor of the coaxial cable.
The measurement chain in U.S. Pat. No. 4,067,225 includes a demodulator that is connected to the output of the first amplifier, a low pass filter, another amplifier, a variable gain amplifier, a peak-to-peak detector and an indicating instrument for measuring the distance between the sensor face and a conductive part or body.
In operation, the output voltage of the first amplifier will bear a ratio the input voltage of the first amplifier that is in proportion to the capacitance ratio between the standard capacitor and the capacitor defined by the space between the central sensor electrode and the conductive body, the distance of this space being to be measured. Since the output voltage of the oscillator is constant and known, the current through the standard capacitor is constant and the voltage at the output of the first amplifier will be inversely proportional to the capacitance of the sensor capacitor, and hence directly proportional to the distance to be measured.
Another distance/capacitance measurement chain that also includes a reference voltage oscillator coupled through a standard or reference capacitor to the input of an amplifier that is connected to the capacitive sensor, is also described in U.K. patent No. 1,047,600.
Nevertheless, the measuring voltage outgoing from the prior measurement chains is not precisely stabilized on temperature and time. The measuring voltage from which the distance is deduced depends on a quasi-constant current powering the sensor and varying slowly in dependence on temperature and time.